1. Field of the Invention
This invention relates to cross-sectional x-ray imaging systems. In a primary application the invention relates to the correction for spectral energy shift of a polychromatic x-ray source in cross-sectional imaging systems. In another application the invention relates to obtaining cross-sectional x-ray images of specific materials.
2. Description of Prior Art
Recently two commercial instruments, the EMI and ACTA scanners, have been introduced which provide an accurate cross-sectional image of the brain. This is accomplished by measuring the x-ray projections through the head at 180 different angles and using various mathematical techniques to reconstruct the cross-sectional density. This system is described in a paper by J. Ambrose and G. N. Hounsfield in the British Journal of Radiology, vol. 46, 1973.
One of the biggest sources of inaccuracy of these instruments is the energy spectral shift of the x-ray beam as it traverses the various materials in the cross-sectional slice. The log of the measured transmitted intensity should represent the sum of the linear attenuation coefficients along the x-ray beam. This will be the case if a monoenergetic source is used. These sources, however, have insufficient strength to complete the scan in a reasonable time interval. The use of broad-band or polychromatic x-ray sources, which have sufficient strength, results in an accuracy problem since the attenuation coefficients are a function of energy. For example, if a small volume element is traversed by an x-ray beam from two different angles it can provide a different attenuation coefficient at each angle. If the x-ray beam at the different angles goes through different amounts or different types of material it will have a different resultant energy spectrum which in turn results in different attenuation coefficients. The EMI scanner attempts to minimize this problem by using a water bag around the head so that every beam will have the same path length. The relatively uniform geometry of the skull minimuzes the problem of different amounts of bone in the path of the x-ray beam. Thus the EMI scanner is able to achieve reasonably high accuracies while using a broad-band x-ray source. This is described in detail in, "An Evaluation of the Quantitative and Radiation Features of a Scanning X-Ray Transverse Axial Tomograph" by E. C. Mccullough, et.al. in Radiology, vol. 111, June 1974, on pages 709-715.
In many applications the shift of the broad-band energy spectrum is a serious problem. In most areas of the body, outside of the upper region of the head, the beams at different angles encounter significantly different amounts of bone in the path which would result in serious inaccuracies in a reconstructed cross section. The ACTA scanner has discarded the path-length compensation water bag for convenience, although they have not published data on their resultant accuracy.
In addition to obtaining accurate density images, it would be desirable to delineate specific materials within the cross section. This can be accomplished by making x-ray transmission measurements at more than one energy spectrum since different materials have varying attenuations to different regions of the x-ray spectrum. The only published attempt to do this was by J. Ambrose and G. N. Hounsfield in the previously listed paper in the British Journal of Radiology, vol. 46, 1973. In this case two cross-sectional images were made at 100 and 140 Kv. accelerating voltage as the mechanism to change the energy spectrum. Despite the fact that this represents a relatively subtle spectral change, relatively small amounts of iodine were able to be distinguished from calcium densities.